1. Field of the Invention
This invention relates to a pattern recognition apparatus for determining the probability that the shadow of a tumor on a radiation image is the shadow of a malignant tumor, based on an image signal representing the radiation image.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen for the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal, and the electric signal (image signal) is processed and then used for reproducing the X-ray image as a visible image on a copy photograph or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as a human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and the image signal is used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT), or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light emitted by the stimulable phosphor varies over a wide range and is proportional to the amount of energy stored during its exposure to the radiation, it is possible to obtain a visible image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain a desirable image density, an appropriate read-out gain is set when the emitted light is being detected with a photoelectric read-out means and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device such as a CRT.
In the above-mentioned systems using the X-ray film or stimulable phosphor sheets, specifically in system constituted for the purpose of facilitating medical treatments or diagnoses, not only have visible images been obtained which have good image quality and are suitable for use in making medical observations (diagnoses), but also, as disclosed in U.S. Pat. No. 4,769,850, for example, pattern recognition of images has been carried out recently.
In this specification, the term "pattern recognition of images" or simply "pattern recognition" shall be understood to mean an operation for extracting a predetermined pattern from very complicated images. For example, the pattern recognition may mean the operation with which the shadow of a tumor is extracted from a lung X-ray image including various linear and circular patterns.
In a complicated image, such as an X-ray image of a lung, mamma, or the like, predetermined patterns, such as shadows of tumors are extracted, and thereafter a visible image wherein the extracted patterns are pointed out is reproduced in order to facilitate observation of the tumor, or the diagnosis thereof by a medical doctor.
In said U.S. Patent No. 4,769,850, a pattern recognition apparatus is disclosed, wherein the image signal corresponding to an X-ray image of a human body is passed through a specific spatial filter so as to extract circular and linear patterns, which are respectively regarded as shadows of tumors and shadows of blood vessels, and then these patterns are displayed.
Radiation images of the human body are very complicated even for a normal (healthy) human bodies. Further, the shadows of tumors which may appear in an X-ray image of a lung, mamma, or the like may have a variety of sizes, and also the shape and density of the shadows may vary according to their position on the image. Therefore, in order to extract a variety of tumor shadows, a circular pattern extracting filter for extracting tumor shadows, for example, the one described in said U.S. Pat. No. 4,769,850, must have a considerably wide tolerance.
However, only the shadows of malignant tumors should be extracted, but if a tumor extracting filter having a wide tolerance is used, both the shadows of malignant tumors and the shadows of benign tumors are extracted.